Elements of Structural and Systematic Botany

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Pond Scums

surface of every stagnant pond and ditch, being commonly known as "pond scum." The commonest of these pond scums belong to the genus Spirogyra, and one of these will illustrate the characteristics of the order. When in active growth these masses are of a vivid green, and o

l section, and the chlorophyll bands are omitted, n, n?, the two nuclei. C, a complete cell. n, nucleus. py. pyrenoid. D, E, successive stage

y different in structure from the ordinary protoplasm, as is shown by extracting the chlorophyll, which may be done by placing the plants in alcohol for a short time. This extracts the chlorophyll, but a microscopic examination of the decolored cells shows that the bands remain unchanged, except for the absence of color. These bands are flattened, with irregularly scalloped margins, and at intervals have rounded bodies (pyrenoids) imbedded in them (Fig. 18, C, py.). The pyrenoids, especially when the plant has been exposed to the light for some time, are surrounded by a circle of small granules, which become bluish when iodine is applied, showing them to be starch. (To show the effect of iodine on starch on a large scale, mix a little flour, which is nearly all starch, with water, and add a little iodine. The starch will immediately become colored blue, varyin

being very rapid. As the cells break apart, the free ends bulge strongly,

ng, but may sometimes be met with at other times. The masses of fruiting plants usually appear brownish colored. If spores h

the point of contact, the wall is absorbed, forming a continuous channel from one cell to the other. This process usually takes place

onnected, the tubes being formed at the end of the cells (Fig

rved.) The young spore thus formed contracts somewhat, becoming oval in form, and soon secretes a thick wall, colorless at first, but afterwards becoming brown and more or less opaque. The chlorophyll bands

ace?. A, Zygnema. B, C,

star-shaped chloroplasts in each cell, and Mesocarpus (Fig. 19, B, D), in which the single chloroplast has the form of a thin median plate. (B shows the appearance from in front, C

Cosmarium. D, and D? show the process of division. E,

cur in masses of sufficient size to be seen with the naked eye, usually being found associated with pond scums or other filamentous forms. Many of the most beautiful forms ma

egant outlines. The cell shows a division into two parts, and is often constricted in the middle, each division hav

ht green, except at the ends and in the middle. The large chloroplast in each half is composed of six longitudinal plates, united at the axis of the cell. Several large pyreno

or creeping slowly over the slide as we examine them,

on they closely res

IV.-Si

re those belonging to the genus Vaucheria, but these are to be had almost everywhere. They usually occur in shallow ditches and ponds, growing on the bottom, or not infrequently becoming free, and floating where the water is deeper. They form large, dark

nd to be denser, this being the growing point. By careful focusing it is easy to show that the protoplasm is confined to a thin layer lining the wall, the central cavity of the tube being filled with cell sap. In the protoplasm are numerous elongated chloroplasts (cl.). and a larger or smaller number o

. og. o?gonium. D, a ripe o?gonium. E, the same after it has opened. o, the egg cell. F, a ripe spore. G, a species i

ower end is colorless, and forms a more or less branching root-like structure, fastening

multiplies non-sexually by branches pinched off by a constriction at the point where they join the main filament, or

anches, and their arrangement is such as t

mes club-shaped, and the end somewhat pointed and more slender, and curves over. This slender, curved portion is almost colorless, and is soon shut of

shut off, a circle of buds appears about its base (Fig. 21, B, og.). These are the young o?gonia, which rapidly increase in size, assuming an oval form

ver, so very small as scarcely to be visible except with the strongest lenses. They are littl

on in Vaucheria sessilis. B, non-

ontents is forced out, leaving an opening, and at the same time the remaining contents contract to form a round mass, the germ or egg cell (Fig. 21, E, o). Almost as soon as the o?gonium opens, the spermatozoids collect about it and enter; but, on account of their minuteness, it is almost impossible to follow the

organs are borne directly o

parated by a wall from the rest of the branch, the end opens, and the contents escape as a very large zo?spore, covered with numerous short cilia (A ii). After a short period of activity, this loses its cili

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are so very different from the other green alg? that it

ime found in most of them, giving them a harsh, stony texture. Several species are common growing upon th

etween the nodes (internodes) may in the larger species reach a length of several centimetres. The leaves are slender,

o small, roundish bodies, in the commoner species of a reddish color (Fig. 23, A, r). The

whole plant is harsh to the touch and brittle, owing to the limy coat

internode. E, of a node of a somewhat older leaf. F, G, young sexual organs seen in optical section. o, o?gonium. An. antheridium. H, superficial view. G, I, group of filaments containing spermatozoids. J, a sma

, S). This from its position is called the "apical cell," and from it are derived all the tissues of the plant. Segments are cut off from its base, and these divide again into two by a wall p

he former, as by cutting several cross-sections of the whole bunch of young leaves near the top of the pl

oth from above and below, meeting in the middle of the internode and completely hiding the long axial cell. A section across t

show a very marked movement of the protoplasm. These movements are best seen, however, in forms like Nitella, where the long int

d, and the apical cell becomes finally very long and pointed (Fig. 23, C). In some species the chlo

of special branches that may become detache

sing in pairs at the joints of the leaves. The

becomes divided into an upper and a lower one, the latter becoming twisted as it elongates, and the central cell later has a small cell cut off from its base by an oblique wall. The central cell forms the egg cell, which in the ripe o?gonium

e centre. In each of these eight cells two walls are next formed parallel to the outer surface, so that the antheridium (apart from the basal cell) contains twenty-four cells

the inner cells, the second series, while not increasing in diameter, elongate, assuming an oblong form, and from the in

ing together by deeply indented margins, and having the oblo

are much larger than any we have met with. Each is a colorless, spiral thread with about three coils, one end being somewhat dilated with a few granules; the other

nium become very hard, and the whole falls off,

ng plant consists at first of a row of elongated cells, upo

Nitella and Tolypella. The second family have the internodes without any cortex-that is, consisting of a single long cell; and the c

ive organs, especially the very complex antheridium, the Charace? are very